This application is based upon and claims priority of Japanese Patent Application No. 2000-369323, fled, the contents being incorporated herein by reference.
The present invention concerns an improved method for forming an opening which possesses a step differential patternwise.
Prospects of practically mass-producing Cu (copper) wire layers in place of extant Al (aluminum) wire layers have come to be vigorously investigated from the standpoint of lowering the intrinsic electrical resistance values of wire layers, which has been become urgent in response to the size reductions of wire layers, which inevitably accompany attempts to reduce the sizes of semiconductor devices. Unlike the Al (aluminum), however, the Cu (copper) is an extremely difficult material to process due to production process-related limitations, and it is indispensable to develop unprecedented techniques for both blanketwise formation processes and patterning processes, and accordingly, a so-called xe2x80x9cdual damascene structure,xe2x80x9d which possesses a downwardly protruding cross-sectional shape, has come to be proposed as the shape of an opening for configuring a wire layer within an interlayer insulating film.
The present invention provides a technique which is most desirable as an improved opening formation process for this dual damascene structure.
First, dual damascene wiring processes of the prior art will be explained with reference sequentially to FIGS. 1 through 6.
FIGS. 1 through 6 show cross-sectional views during the respective processes of the dual damascene wiring technique of the prior all, and cross-sectional appearances of the apparatus corresponding to the respective processes are thereby shown modelwise. The cross-sectional structure of the dual damascene wire layer is characterized by an approximately downward protrusion, and this technique of the prior art may therefore be said to be founded roughly on the following concept. In other words, an interlayer insulating film is first etched halfway by using a resist mask which possesses a large opening, and after it has subsequently been substituted with a resist mask which possesses a smaller opening, a window is formed on the interlayer insulating film in such a way that the segment of the interlayer insulating film the thickness of which has become reduced as a result of the halfway etching alone will become etched through to the lower wire layer.
Next, such a technique of the prior art will be explained in detail below, see FIG. 1.
As far as the dual damascene wiring processes of the prior art are concerned, the impurity electroconductive layer (2), which serves as a wire layer, is first configured within the semiconductor substrate (1), and subsequently, the etching stopper (3), the interlayer insulating film (4), and the resist mask (5) are formed in proper order above it. The resist mask (5) is patterned by using a well-known photolithographic technique. Next, the interlayer insulating film (4) is selectively removed based on a dry etching technique via the window which has been formed within the resist mask (5) photolithographically, as shown in FIG. 2.
In such a case, the etching of the interlayer insulating film (4) is stopped at a stage where the etching depth remains shallow enough not to reach the base underneath completely, as a result of which a depression is configured within the interlayer insulating film (4). The resist mask (5) is subsequently removed by means of ashing, as shown in FIG. 3.
The resist mask (6) is coated anew on the entire plane of the interlayer insulating film (4), including the depression configured earlier. A window is formed on this resist mask (6) based on a well-known photolithographic technique. As FIG. 4 illustrates, the width of the window thus formed is sufficiently narrower than the width of the depression while the step differential imputed to the depression is being bared to its bottom.
Next, the interlayer insulating film (4) is selectively etched by using the resist mask (6). In such a case, the etched segment of the interlayer insulating film (4) includes the step differential, and therefore, in a case where the etching is terminated at the stage where the etching stopper (3) has become bared to the bottom of the interlayer insulating film (4), the step differential shown in FIG. 5, which has fundamentally inherited the traits of the initial step differential, comes to emerge within the window. Subsequently, the resist mask (6) is removed by means of ashing, as shown in FIG. 5.
Next, the etching conditions are redesignated, and the etching stopper (3), which has come to become bared to the bottom of the interlayer insulating film (4), is removed, as a result of which the window becomes etched through to the impurity electroconductive layer (2). Thus a dual damascene window with a downwardly protruding shape is formed in the interlayer insulating film (4), as shown in FIG. 6.
A thin tantalum nitride (TaN) layer (not shown), furthermore, is formed on the inner wall of the dual damascene window by means of sputtering. This layer, which serves as a barrier layer for preventing the diffusion of a copper (Cu) wire layer to be formed later toward the interlayer insulating film (4), is an indispensable element for the copper (Cu) wire constitution of a dual damascene structure. Next, a thin copper (Cu) seed layer (not shown) is formed on the surface of the tantalum nitride (TaN) layer by means of sputtering. This copper (Cu) seed layer serves the function of a seed layer during a plating process whereby the interior of the dual damascene window is completely filled with the copper (Cu) layer (9). A film that constitutes the copper (Cu) layer (9) is formed at a sufficient thickness based on the plating method in such a way that it will bulge from the window initially, but the bulge beyond the dual damascene window is subsequently removed based on the CMP (chemicomechanical polish) method, as a result of which the cross-sectional shape shown in FIG. 6 is achieved.
Thus, the processes of the prior art for forming a copper (Cu) dual damascene wire constitution has been explained.
The dual damascene wiring technique of the prior art is plagued with fatal problems which cannot be overlooked in the context of size reduction, and they will be explained below.
FIG. 7, which corresponds to the process shown in FIG. 3, which has been referred to earlier, is a diagram which shows a cross-sectional view of a state where the process is in progress and which points out the first problem of the prior art. The etching stopper (3) and the interlayer insulating film (4) are formed in proper order above the semiconductor substrate (1), on the surface of which has been formed the impurity electroconductive layer (2) previously, and a depression is formed patternwise within the interlayer insulating film (4) by means of selective etching. In such a case, the interlayer insulating film (4) is not etched through as a result of etching, but rather, the etching is terminated halfway along the thickness of the interlayer insulating film (4) in the context of configuring said depression. Next, the resist mask (6) is formed blanketwise over the entire plane of the interlayer insulating film (4), including the depression, and a subsequent operation for patterning a resist mask opening is carried out based on a well-known photolithographic method. In a case where a resist of the negative type is hereby assumed to be used, exposure beams become scattered under the pervasion of the step differential in the interlayer insulating film (4), and accordingly, patterning irregularities are incurred in the region on which the resist mask opening is to be formed during an operation for transferring a negative pattern. A mask is configured on a plane that includes the step differential in the case of the transfer of the negative pattern, and the step differential segment should ideally remain unexposed to beams. Due to the beam scatters in the vicinity of the step differential, however, the region which should be masked becomes exposed to the beams, which is problematic in that the crucial fringe portions of the resist pattern become significantly irregular. The following problem, however, remains unsolved even in a case where the aforementioned scatters of exposure beams can be inhibited: A chemical amplification-type resist, which is patterned under the pervasion of an acid which has been generated as a result of beam exposure, has come to be used almost exclusively in recent years, but in the case of such a chemical amplification-type resist, the acid which has been generated from an optical acid generator which has been internalized in it becomes consumed by traces of amine, ammonia, etc., which become scattered into the air in the vicinity of the resist coating film surface, which is problematic in that an accurate pattern cannot be formed as a result of a development and that the shapes of the developed pattern tends to become irregular. Such a problem of the pattern morphological irregularity (e.g., sleeve formation, etc.),becomes especially grave in a case where a microscopic device, wherein the proximity between the step differential segment of an interlayer insulating film and a window is high, and as FIG. 7 indicates, the sleeve which has been formed on the resist pattern fringe completely covers the step differential of the interlayer insulating film, which is in turn problematic in that it becomes impossible to open a window even in a case where a dry etching process is implemented. The first problem of the prior art has been thus explained. In a case where this mode of defect arises, the obtained product is obviously unshippable, as a result of which the yield decreases.
FIG. 8, which corresponds to the process shown in FIG. 3, which has been referred to earlier, is a diagram which shows a cross-sectional view of the process in progress and which points out the second problem of the prior art. The respective elements shown and numerically notated in FIG. 8 are identical to the elements which bear the corresponding notations in FIG. 7.
The second problem concerns the problem of a contact resistance gain. Even if the sleeve problem of the resist can somehow be solved, a small window is independently positioned and opened without being aligned against the step differential segment of the interlayer insulating film, and therefore, the proximity between the step differential segment and the window to be opened inevitably becomes high, as a result of which the bottom of the window tends to become small. The case shown in FIG. 8 pertains to one where the comers of the depression configured on the interlayer insulating film (4) are blanketed by the window of the resist mask (6). In such a case, slight positioning imprecisions come to affect the size of the contact plane, namely the magnitude of the contact resistance, and in an extreme case, the yield must be considered to become low. Even if such an extreme can be avoided, the device performances may diminish, or a secondary problem of having to designate and adjust other process conditions more severely may arise. Even in a case where an attempt is made to form a resist mask by forming a small window in the interior of a shallow and large depression which has been formed on the interlayer insulating film in such a way that it will not overlap the fringe plane of the depression (step differential segment), on the other hand, the fact that the resist thereby prevails as a mask for opening a window deep enough for baring the contact plane underneath remains unchanged. Thus, the interior of the interlayer insulating film must be subjected to two etching processes while mutually independent resist masks are being used on the respective occasions, and the residues which have been emitted from the resists which serve as the etching masks and which have become adhered to the surface must be removed twice. Such residues are normally removed by means of chemical solution treatments, and in a case where such treatments are performed twice, the prospects of the chemical solution becoming significantly absorbed into the films which constitute the respective layers and of unfavorable consequence being incurred as a result of its scatter during a subsequent thermal treatment become more likely. The initial etching operation for configuring the depression is also problematic in that it is difficult to control the etching magnitude for forming the designed dual damascene structure by terminating the etching halfway in the midst of the interlayer insulating film without reference to a clear-cut etching end point. The second problem of the prior art has been thus explained.
As the foregoing first and second problems shown in FIGS. 7 and 8, respectively, which have been explained individually, suggest, it is generally difficult to secure a sufficient contact area based on the processes of the prior art for manufacturing a dual damascene wire structure, and in an extreme case, a complete failure to open a window, which is a fatal flaw, may be incurred. It is inevitable for the consequence of such an inconvenience to become more grave as the device size decreases, and therefore, it has become urgent to develop a novel technique for manufacturing a dual damascene wire structure, wherein a sufficient contact area can be secured.
The constitutions shown below are provided by the present invention as mechanisms for solving the aforementioned problems of the prior art.
That is, the present invention has adopted a process for transferring a step differential which has been configured on a hard mask onto an interlayer insulating film underneath in the context of forming a dual damascene structure. In other words, a locally shallow section is configured on a hard mask, and the shallow portion is initially depleted selectively and partially. Subsequently, the entire plane of a region of the hard mask pattern to be transferred to be base underneath is etched. A downwardly protruding dual damascene structure can in effect be formed in this method based solely on the etching rate differential between the interlayer insulating film and the hard mask formed above it. A contact plane, furthermore, is opened after the removal of the resist, and therefore, the problems of the prior art attributed to resist residues (e.g., contact defect, etc.) can first be eliminated. Even in a case where a microscopic and narrower dual damascene structure is prepared, furthermore, a window can be smoothly opened. The resist has been the window width determining factor in the prior art, and in an extreme case, it may become impossible to open a window even under the pervasion of a slight sleeve, or the problem of a failure to open a window of a desired width may arise. These problems can be utterly eliminated in the present invention, which has adopted a hard mask transfer modality. An attempt was made in the prior art to alleviate the problem of the resist sleeve by opening a mask window while it is being aligned against a step differential plane, but in a case where such a process is implemented, a failure to open a window completely or a failure to open a window of a desired width becomes inevitable unless the mask and step differential are accurately positioned and aligned. As far as the present invention is concerned, on the other hand, there is no need to mutually position and align the resist pattern and the shallow portion within the hard mask accurately, and so long as a window becomes opened at any position within the depression, neither a resistance value variation of a wire to be formed on it later nor a contact resistance value variation is incurred, which is beneficial in that a dual damascene wire structure can be formed with relative ease without recourse to a precise positioning and aligning operation.